This invention relates to a system for and a method of sensing or measuring an electric field with a minimum distortion or perturbation of the electric field being measured. This invention is particularly well-suited for measurement of high voltage electric field such as may be encountered in lightning simulation tests or the like.
Modern aircraft and aerospace vehicles oftentimes must operate in high voltage electric field environments, such as when flying through thunderstorms or the like. The exact effects of high voltage electric field environments and in particular the effects of actual lightning strikes on aircraft are not well known. It is, however, generally known that lightning strikes on aircraft have caused problems in various onboard digital systems, such as digital navigation systems, onboard computers, and other flight safety related equipment. In certain advanced vehicles, conventional servo-hydraulic flight control systems are being replaced by digital flight control systems (referred to as fly-by-wire flight control systems) in which electrical (digital) control signals are transmitted from a pilot controlled input to processors located adjacent the various control surfaces on the aircraft. These processors then control a hydraulic actuator in response to the digital input commands. Usually, an onboard computer is used in a fly-by-wire system to generate these control signals and no conventional servo-hydraulic system is provided as a back-up. The effects of induced voltages and/or currents in fly-by-wire flight control systems are not well-known and thus there is a need to be able to test these systems in high voltage electric field environments and when the aircraft is struck by lightning. It is a concern that induced voltages and current in these fly-by-wire systems may cause various problems (e.g., instability or the like) in the fly-by-wire control system.
In general, high voltage electric field tests have been conducted by generating a high voltage field by means of a high voltage generator, such as a staged capacitor Marx generator or the like, by exposing a test specimen to the field, and by measuring the voltages or currents induced in a pair of conductors inside the test specimen. By increasing the voltage of the electric field, arcs simulating lightning strikes may be caused to jump from a high voltage electrode to the test specimen. Addtionally, discharges through the aircraft to the ground (ground return strokes) are simulated by an arc from the test specimen to a ground electrode, These high voltage generators are capable of generating electric field potentials from a few thousand volts to several thousand or even a few million volts.
In other types of lightning simulation tests, a low level pulsed current test is performed which is sometimes referred to as a lightning transient analyzer test. This technique employs the use of low level current pulses which are transmitted through the aircraft structure (i.e., the test specimen) so as to produce transient signals on internal wiring. The transients on selected circuits are measured and interpreted (i.e., extrapolated upwardly) in terms of the parameters of the driving current wave form. This technique has been useful for rapidly screening various aircraft circuits. However, this technique requires the assumption that all coupling is magnetic and it further require extrapolation of the data over a extremely wide range.
A long standing and important problem has been persistently present in conducting high voltage electric field tests, namely, how to reliably and accurately measure the strength of the electric field. It is a typical requirement of most high voltage electric field tests to determine not only the induced currents within the test specimen, but also to determine the strength of the electric field to which the test specimen is exposed and to have a relatively high level of confidence in these electric field strength measurements. According to the basic principles of electrostatics, however, any conductive body, whether charged or uncharged, modifies an electric field by its mere presence in the field because of the effect broadly known as electrostatic induction. Generally, when any conductive body is exposed to an electrostatic field, it becomes polarized (i.e., the positive and negative charges within the body tend to move within the body). This polarization causes the overall electric field to be altered from what it would have been if the body had not been present in the electric field.
Not only does a sensor placed in the electric field distort the field, but the sensor must, of course, be coupled to a read-out device, such as an oscilloscope, a recorder, or a data acquisition system, so that the output of the sensor may be observed and recorded. However, if the read-out device is placed in the electric field, its mass will also effect the field to an even greater degree than the sensor alone. If the read-out device is located remotely from the sensor, the lead wires connecting the sensor and the read-out device will, in some instances, form a ground loop and thus alter the field.
Likewise, another similar problem with recording test data from the test specimen is known. More specifically, ground loops between the test specimen and the remotely located data acquisition systems have resulted in perturbations of the electric field surrounding the test specimen and thus have adversely affected the results of the tests.
Accordingly, test personnel could not be certain whether the field strength measurements being recorded realistically represented the field to which the test specimen was actually exposed. Moreover, prior sensors tended to distort the field to such a degree that many changes in field strength could not even be measured. As a result of this inability to measure many changes in the electric field, the actual response of test specimens in rapidly changing high voltage fields could not heretofore realiably be determined.
While field measurement techniques are also quite important in such test areas as radio frequency interference (RFI) and nuclear electromagnet pulsation (EMP) test techniques, these other areas of electric field measurement do not involve the high voltage fields used in lightning simulation testing. Thus, the special problems encountered and solved in the RFI and EMP test environments have generally not been applicable to high voltage electric field testing.
Additionally, accurate data of the electric field characteristics of actual thunderstorms and cloud formations is not well-known. One of the problems in making accurate assessments of actual electric field data in thunderstorms and cloud formations is that the airplane entering the area to be measured distorts the electric field therein. Thus, there is a need to know the actual electric field environment of a thunderstorm or other atmospheric phenomenon so that the electrostatic field of thunderstorms and lightning strikes can be more accurately simulated.